1. Field of the Invention
The present invention relates to a fixing device for an image forming apparatus utilizing electrophotography, such as copiers, printers, facsimiles, etc., and more particularly to a belt fixing device and a method for controlling the fixing temperature of the device in a stable manner.
2. Discussion of the Related Art
As a fixing device for an image forming apparatus utilizing electrophotography, there is well known a heat roller fixing device that includes a fixing roller having an internal heat source and a pressing roller pressed against the fixing roller. A transfer sheet carrying an unfixed toner image thereupon is held by and conveyed through a fixing nip formed between the fixing roller and the pressing roller and thereby the toner image is fixed onto the transfer sheet. The toner is fixed on the transfer sheet by heat applied from the fixing roller and pressure applied by the rollers.
As a method of controlling the fixing temperature of such a heat roller fixing device, an example is disclosed in Japanese patent laid-open publication 1-282585. There, the temperature of a fixing roller is set higher immediately after warm-up than when the device is in a stable condition, and the temperature of the fixing roller is controlled so as to gradually fall as time elapses or as the number of transfer sheets passing through the device increases.
In the heat roller fixing device above, toner is melted to be fixed onto a transfer sheet contacting a surface of the fixing roller which is heated by the heat source. Because the heater is disposed inside of the fixing roller, control of the surface temperature of the fixing roller to a desired temperature for fixing is done in a stable manner. On the other hand, because the fixing nip is formed by the rollers contacting each other, the length of the fixing nip (the distance in the transfer sheet conveying direction) is inherently limited. Therefore, when the temperature for fixing is made low for reducing consumption of electric energy, a longer fixing time (a time during which a transfer sheet is conveyed through the fixing nip) is needed for satisfactory fixing. Consequently, a linear speed of the fixing and the pressing rollers has to be slower. Therefore, the device is not suitable for use in a high speed image forming apparatus. Further, because the surface of the fixing roller is always kept at a high temperature, heat is supplied to the surface of the fixing roller from the internal heat source and, as the fixing time becomes longer, the temperature of toner carried on a transfer sheet rises rather high, and consequently the temperature where offsetting of transferred toner starts to occur becomes relatively low. This reduces the tolerance of a fixing temperature (a range of fixing temperatures in which toner can be satisfactorily fixed and offsetting of the toner does not occur).
For solving the above problem of a heat roller fixing device, there is proposed a belt fixing device using a belt. A belt fixing device disclosed in Japanese patent laid-open publication 4-273279 uses none or a very small quantity of a release agent, such as a silicon oil. The device accomplishes satisfactory fixing (melting of toner) and a well reproduced color image superior in glossiness. The belt fixing device includes an endless belt and a pair of rollers which are pressed to each other while interposing the belt therebetween. The belt is spanned around one of the pair of rollers and another roller which is arranged apart from the one of the pair of rollers by a certain distance. A heater for heating the belt is disposed inside one of the pair of rollers. A transfer sheet carrying an unfixed toner image thereupon passes through the pair of rollers and along a part of the belt and thereby the toner image is fixed onto the transfer sheet. The endless belt includes a layer of a release agent on a base member of the belt. A temperature detecting device is provided in a position opposing a circumferential surface of the roller having the internal heater and where the belt is not in contact with the roller, and also in a position opposing a circumferential surface of the other roller pressed against the heating roller via the belt.
Another example of a belt fixing device disclosed in Japanese patent laid-open publication 4-362984 can fix a toner image without disturbing the image and can precisely control the fixing temperature relative to a transfer sheet without damaging a fixing roller or a fixing belt. The device is also capable of obtaining optimum fixing conditions for each transfer sheet having a different thickness according to the thickness of the sheet, without changing the conveying speed of the transfer sheet. The device includes a fixing member made of a thin seamless metal, having a release agent layer formed on the surface and being formed as an endless belt. The belt is spanned around a heating roller including an internal heat source and at least another roller, and is arranged along a circumferential surface of a pressing roller made of a rigid body or a rigid body having a thin elastic member on the surface, so as to form a fixing nip at the position where the belt contacts the pressing roller. Further, a temperature detecting device for detecting the fixing temperature for a transfer sheet is provided and the heat source is controlled based upon the result of detecting the temperature for a transfer sheet by the temperature detecting device.
In the above belt fixing device disclosed in Japanese patent laid-open 4-273279, first a toner image is fixed via the belt when a transfer sheet passes between the pair of rollers. The sheet is then conveyed by the belt and is then separated from the belt. Therefore, although wrinkling of a transfer sheet and rubbing of an unfixed toner image on the transfer sheet rarely occur, conveying of the transfer sheet after fixing is unstable, glossiness of an image tends to be uneven and further offsetting of transferred toner tends to occur. More particularly, a transfer sheet and toner thereupon receive the remaining heat of the belt after passing through the nip portion and thereby the toner excessively melts to cause an offsetting phenomenon. Further, unstable conveying of the transfer sheet causes uneven temperature distribution on the surface of the toner image, which results in uneven glossiness in the image.
In the fixing device of Japanese patent laid-open publication 4-362984, wrinkling of a transfer sheet rarely occurs because the fixing nip is formed by curving the belt along the circumferential surface of the pressing roller and as a result the pressing force to the transfer sheet is weak. On the other hand, because of weak pressure, it occurs that fixing is not satisfactory. Further, when a velocity difference occurs between the belt and the pressing roller, an unfixed toner image on a transfer sheet is rubbed by the belt and is disturbed
For solving the above described problems, the inventors of the present invention proposed a belt fixing device for a color image forming apparatus which does not use oil as a release agent and which prevents occurrence of a toner offsetting phenomenon and reduces a warm-up time. FIGS. 1 and 2 are a schematic drawing illustrating a construction of this (non-prior art) device. The belt fixing device includes a fixing roller 2, a heating roller 1, an endless fixing belt 3 spanned around the fixing roller 2 and the heating roller 1, and a pressing roller 4 disposed opposing the fixing roller 2 via the fixing belt 3. A heater 5 for heating the fixing belt 3 is provided at least in the heating roller 1. The fixing device is provided with a heating area 12 where the fixing belt 3 is heated by the heating roller 1 and a fixing area including a first fixing part 8 where the pressing roller 4 contacts the fixing belt 3 without pressing the fixing roller 2 and a second fixing part 9 where the pressing roller 4 presses the fixing roller 2 via the fixing belt 3.
In this fixing device, the heating area 12 for heating the fixing belt 3 is provided independently from the fixing area for fixing a toner image on a transfer sheet 13. The belt 3 is heated by the heating roller 1 in the heating area 12. When the heated part of the belt 3 passes through the fixing area, the belt 3 first heats a transfer sheet 13 carrying an unfixed toner image thereupon at a fixing nip of the first fixing part 8. The belt 3 then carries the transfer sheet 13, and the toner image is fixed onto the transfer sheet 13 when heat and pressure are applied to the toner image at a fixing nip at the second fixing part 9. Therefore, stable fixing, prevention of toner offsetting and reduction of the warm-up time are accomplished by optimally controlling the pressure between the fixing belt 3 and the pressing roller 4 at the first fixing part 8, the pressing force by the pressing roller 4 at the second fixing part 9 and the heat capacity of the fixing belt 3.
However, the above device has a problem in that the fixing temperature is unstable when starting a print and when prints are continuously made, which will be explained next more in detail. For controlling the fixing temperature, the belt fixing device in the above Japanese patent laid-open publication 4-273279 includes a thermistor as a device for detecting the temperature of the belt in a position opposing a circumferential surface of the fixing roller disposed inside of the endless fixing belt and another thermistor in a position opposing a circumferential surface of the pressing roller being pressed against the fixing roller via the belt. A heat source is arranged in either the fixing roller or in the pressing roller, and the fixing temperature is controlled by controlling the heat source based upon the detected result by the thermistors.
In the fixing device disclosed in Japanese patent laid-open publication 4-362984, thermistors are arranged so as to contact a surface of the heating roller and the back side of the fixing belt at the fixing nip portion, respectively, and heat sources respectively disposed in the heating roller and the pressing roller are activated and controlled at the same time.
In the above devices, the thermistor is provided to contact or to oppose a surface of the heating roller having the internal heat source, which is disposed inside of the endless fixing belt, and the surface temperature of the fixing belt is indirectly controlled by controlling the heat source in the heating roller or in the fixing roller. In this configuration, because the heat transmission property of a fixing belt varies depending upon the heat capacity of the fixing belt, it is very hard to always stably maintain the surface temperature of the belt at a certain level. This problem becomes significant when the printing speed increases or when color images are fixed.
An example of controlling the fixing temperature in the above belt fixing device is now explained. FIG. 3 shows a relationship between the print time and the temperature on a surface of the fixing belt (hereinafter the fixing belt temperature) when prints are continuously made in an image forming apparatus using the fixing device shown in FIGS. 1 and 2. The temperature is measured at a substantially middle point of the heating area 12 (where the transfer belt 3 is in contact with the heating roller 1) in the rotating direction of the belt 3, which point is a center point of the heating roller 1 in the axial direction and corresponds to the position of the thermistor 7 in FIG. 1, and also at the exit of the fixing nip portion (the exit of the fixing area).
As shown in FIG. 3, when the fixing belt 3 is stopped as in a waiting mode, the temperature of the fixing belt 3 at the middle point of the heating area 12 is controlled by the heating roller 1 to a prescribed temperature such as about 150.degree. C. When printing is started and the fixing belt 3 starts to rotate, because a surface of the fixing belt 3 contacts the pressing roller 4 at the fixing nip portion in the fixing area and at the same time the inner surface of the fixing belt 3 contacts the fixing roller 2, the fixing belt 3 loses heat and consequently the belt temperature at the middle point of the heating area 12 rapidly falls, for example, to about 130.degree. C. Then, as the rotating time of the fixing belt 3 (the printing time) elapses, the temperature of the pressing roller 4 and the fixing roller 2 rises due to heat supply from the fixing belt 3. With the temperature rise of the pressing roller 4 and the fixing roller 2, the temperature of the belt 3 at the fixing nip portion gradually rises and reaches the same temperature as the belt temperature at the middle point of the fixing area 12 (for example, about 150.degree. C.). Here, the belt temperature at the fixing nip portion is replaced by the belt temperature at the exit of the fixing nip portion because measuring the belt temperature at the fixing nip portion is very hard and further because it is generally required that the belt temperature at the exit of the fixing nip portion, which is the closest position to the fixing nip portion where an unfixed toner is melted to be fixed onto a transfer sheet, is higher than a toner melting point for satisfactory fixing. In the following description also, reference to the belt temperature at the fixing nip portion will actually refer to the belt temperature at the exit of the fixing nip portion.
Under the above conditions, because the belt temperature at the exit of the fixing nip portion rises as the rotating time of the fixing belt 3 elapses, the fixing qualities obtainable with the device, such as the fixing property, the glossiness of an image or the toner offsetting property, change when prints are continuously made. This results in images of uneven quality. Further, when the continuous printing time is long, the belt temperature at the exit of the fixing nip portion rises and the temperature exceeds an upper limit of the belt temperature for fixing at the exit of the fixing nip portion, such as, for example, about 135.degree. C., which is close to the belt temperature at the middle point of the heating area 12. If the belt temperature exceeds such an upper limit, a toner offsetting problem occurs. Therefore, for solving the above problems, it is preferable that the belt temperature at the exit of the fixing nip portion is maintained within a prescribed range of temperatures suitable for fixing.
Therefore, there exists a need for a belt fixing device for use in an image forming apparatus that controls the temperature of a fixing belt at the exit of a fixing nip portion within a range of temperatures suitable for fixing in a stable manner under any conditions while prints are being made so that images having an uniform fixing quality are produced. Particularly, there exists a need for a belt fixing device capable of maintaining uniform glossiness of color images and preventing a toner offset when prints are continuously made.